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(Sorry for bad english, please.)

M. Postnikov in his lections on geometry write that the theorem

Sphere $S^{n-1}$ isn't retract of ball $B^n$.

calls "Drum theorem" because for $n=2$ it mean that we can stretch a film over a circle and make drum.

But I don't understand. Why?

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Well, if the Disk would retract to the circle, the drum would not drum if you drum it. The material would flow smoothly to it's border – Blah Mar 16 '12 at 10:58
@Blah: Cool! Thanks! It's so simple... – user14284 Mar 16 '12 at 11:10

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